The fiber optics telecommunications (“telecom”) field includes such technologies as fiber optical cables and fiber optical networks. Fiber optical networks carry a great variety of everyday information signals, such as conversations, data communications (e.g., fax messages), computer-to-computer data transfers, cable television, the Internet, and so forth. Such information signals are transported between cities as well as from place to place within cities. Due to rapidly increasing communications traffic, the increased capacity of fiber optical cables is more and more necessary, compared to the lower capacities of older metallic wire cables.
An optical fiber cable is typically composed of a bundle of individual optical fibers. One single optical fiber can carry thousands of data and communication signals on a single wavelength of light. That same single optical fiber can also carry multiple wavelengths of light, thus enabling it to carry many, many multiple optical signals at the same time. Each wavelength alone can carry data that transfers at a rate over 10 Gbit/s.
To lay out such optical networks and maintain their communications, it is necessary to perform a variety of sensitive analyses, such as measuring both the chromatic and polarization dispersion, monitoring the optical power, wavelength, and the optical signal-to-noise ratio of the optical signals at each of the wavelengths traveling through the optical fiber, and so forth. Traditionally, such analyses are carried out by several analytical tools including chromatic dispersion (“CD”), polarization mode dispersion (“PMD”), and optical spectrum analyzers (“OSA”).
Chromatic dispersion means that light with different wavelengths travels though the media with different speeds. An optical pulse, which consists of different wavelength components, will be broadened as it travels through the optical fiber due to CD. There are several ways to measure CD. One type of CD performs chromatic dispersion analysis based on measuring the relative time delay between optical signals with different wavelengths.
Polarization mode dispersion means that light with different polarization modes travels through the media with different speeds. An optical pulse, which consists of different polarization modes, will be broadened as it travels through the optical fiber due to PMD. There are several ways to measure PMD. One of the methods is called “Fixed Analyzer method”. Using this method, both the full optical power distribution and that of a particular polarization mode distribution of the optical signal are measured and compared, and PMD is derived from counting the peaks of the ratio of the optical power spectrum.
An OSA performs optical spectrum analysis (also referred to as “OSA”), which, as indicated, is the measurement of optical power as a function of wavelength. OSA applications include testing laser and/or light-emitting diode (“LED”) light sources for spectral purity and power distribution, monitoring an optical transmission system of a wavelength division multiplexing (“WDM”) system for signal quality and noise figures, testing transmission characteristics of various optical devices and components, and so forth.
OSA is typically performed by passing an optical signal to be analyzed through a tunable optical filter. “Tunable” means that the filter can be adjusted to resolve or pick out the individual components (wavelengths) of the optical signal.
Three basic types of filters are widely used to make OSAs: diffraction gratings, Fabry-Perot (“FP”) filters, and Michelson interferometers. A tunable FP filter (“TFPF”) has many advantages for OSA. Principal among these are its relatively simple design, small size, fast speed, ease of control, and its great sensitivity for distinguishing optical signals that are very closely spaced (i.e., signals that have frequencies or wavelengths that are very nearly the same.)
Lensed tunable FP interferometers (“FPIs”) have long played an important role in optical spectrum measurements in physics, chemistry, astronomy and other diverse scientific fields. Miniature lensed FPIs adapted to fiber optical systems can provide medium resolution tuning (finesse=100). Lensless fiber FPIs (“FFPIs”), however, can perform at resolutions greater than 500 for tuning functions in optical fiber systems. Such high performance tunable FFPls have enabled interrogator systems for accurately measuring wavelength responses of passive or active fiber optics devices.
The optical resolution of an OSA is the minimum wavelength spacing between two spectral components that can be reliably resolved. To achieve high optical resolution, the filter should have a sufficiently narrow 3-dB bandwidth (“BW”). Additionally, for many measurements the various spectral components to be measured are not of equal amplitudes, in which case the BW of the filter is not the only concern. Filter shape, which is specified in terms of the optical rejection ratio (“ORR”) at a certain distance (e.g., ±25 GHz) away from the peak of the transmission, is also important. Examples include measuring of side-mode suppression of a distributed feedback (“DFB”) laser, and measuring the optical signal-to-noise ratio (“OSNR”) of the various wavelength channels in WDM optical communications systems.
The wavelength scanning range of the FP filter OSA is limited by its free spectrum range (“FSR”). For the same finesse value, the FP filter's BW is proportional to its FSR, which means the larger the FSR, the larger the BW and the poorer the resolution. Thus for many FP filter OSA applications, there are two major challenges. One challenge is to achieve a very high dynamic range for optical signal-to-noise ratio (“S/N”) measurements (for example, for characterizing a dense wavelength division multiplexing (“DWDM”) system). The other is to achieve a very wide scanning range of wavelengths (for example, from 1260 nm to 1640 nm) while maintaining a sufficiently narrow bandwidth. Enhancing the OSNR should not compromise the wavelength scanning range. Enhancing the wavelength scanning range should not compromise the OSNR. The real challenge is to achieve a higher OSNR and a broader wavelength at the same time.
Known FP filter OSAs have a limited wavelength scanning range due to the filter's FSR, which is the spectral separation between adjacent FP orders (optical orders) that can be tuned without overlap. FSR is inversely proportional to the cavity length of the FP filter. By reducing the cavity length, the FP filter can have a very large FSR. By increasing the cavity length, the FP filter can have a very small FSR. A FP filter's FSR is also proportional to the mathematical product of the filter's BW and its finesse. For the same finesse value, by increasing the cavity length and thus reducing the FSR, we can fabricate a FP filter with a very narrow 3-dB BW, thus providing very good spectral resolution. If the BW becomes smaller, the finesse needs to be larger to maintain the same FSR. For the same finesse value of the FP filter, the larger the FSR, the larger the BW. This is not desirable in many applications since the larger the BW, the poorer the spectral resolution. Thus, in using a FP filter to construct an OSA, the FP filter's FSR will limit the filter's wavelength scanning range.
In many technical situations, precision OSA, chromatic dispersion (“CD”), and polarization mode dispersion (“PMD”) measurements must be made. This requires utilizing multiple instruments, which can be quite inconvenient in temporary or field locations that require the instruments to be carried to the site and then individually connected to local optical networks and individually operated to perform the various measurements. It would therefore be advantageous to combine such OSA, CD, and PMD measurement functionality into a single, multi-functional module. This would not only save substantial component costs by reducing the redundancy of common components (e.g., power supplies), but would also open the possibility of utilizing sophisticated components, such as a FP filter, for all such measurements, resulting in significant cost savings along with significantly improved test instrument performance.
Unfortunately, prior devices have heretofore not been able to effectively, economically, and satisfactorily combine precision OSA, CD, and PMD measurements. For instance, prior devices have been unable to utilize just a single FP filter for all such measurements across the full range of optical communications wavelengths (e.g., from 1260 nm to 1640 nm). One unsolved technical obstacle, for example, has been that such a FP filter not only needs to be operated very precisely across the full optical communications wavelength range, but also needs to be operated in a scanning mode for OSA and PMD measurements, but in a stepping mode for CD measurements.
Thus, a need still remains for high-performance, high precision, integrated measurement systems that can provide and perform OSA, CD, and PMD measurements across the broad wavelength range from 1260 to 1630 nm, for DWDM and CWDM (“coarse wavelength division multiplexing”) applications. A need remains for such systems that can perform such analyses for the full optical communications bands. A need further remains, in particular, for such systems that efficiently provide these functions employing but a single TFPF for the measurement of wavelengths of light. A still further need remains for such single TFPF systems that efficiently provide these functions over such broad, and even further extended, wavelength ranges.
In view of the ever-increasing commercial competitive pressures, increasing consumer expectations, and diminishing opportunities for meaningful product differentiation in the marketplace, it is increasingly critical that answers be found to these problems. Moreover, the ever-increasing need to save costs, improve efficiencies, improve performance, and meet such competitive pressures adds even greater urgency to the critical necessity that answers be found to these problems.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.